This invention relates to machining and, more particularly, to an improved method and apparatus for performing metal working operations such as turning, boring, shaping, grooving, threading and milling.
A cutting tool generally includes a holder and one or more cutting inserts each having a surface terminating with one or more cutting edges. The holder is formed with a socket adapted to receive the cutting inserts which are clamped in a position so that in metal working operations such as turning, boring, shaping, milling, threading and grooving, the cutting edges of the inserts engage a workpiece and remove a chip of metal. The chips comprise a plurality of thin, generally rectangular shaped sections of metal which slide relative to one another along shear planes when separated from the workpiece. This shearing movement of the thin metal sections forming the chip generates a substantial amount of heat in addition to the heat generated by abrasion of the cutting edge of the insert as it contacts the workpiece.
Among the causes of failure of the cutting inserts of tool holders employed in prior art machining operations are abrasion between the cutting insert and workpiece, and a problem known as cratering. Cratering results from the intense heat developed in the formation of the chip, and the frictional engagement of the chip with the cutting insert.
As the metal forming the chip is sheared from the workpiece, it moves along the top surface of the insert and in some cases along the socket portion of the tool holder which secures the insert in place. Many inserts include a chip breaker groove on the surface which faces the chip for turning the chip upwardly away from the insert surface and the socket portion of the tool holder. However, even with chip breaker grooves, at least a portion of the upper surface of the insert inwardly from its cutting edge is in frictional engagement with the chip. Due to this frictional engagement, and the intense heat generated in the formation of the chip, craters are formed on the exposed, upper surface of the insert. Once these craters become deep enough, the entire insert is subject to cracking and failure along its cutting edge, and along the sides of the insert, due to abrasive contact with the workpiece. Cratering has become a particular problem in recent years due to the development and extensive use of alloy steels, super hard alloys such as titanium, stainless and nickel-based alloys.
Prior attempts to avoid cratering and abrasive wear of the insert have provided only modest increases in tool life and efficiency. One approach in the prior art has been to form inserts of high strength materials such as tungsten carbide. Although very hard, carbide inserts are brittle and can be easily chipped which results in premature failure.
To improve the lubricity and strength of inserts, such materials as hardened or alloyed ceramics have been used and a variety of low friction coatings have been developed for coating cutting inserts. Many inserts are currently manufactured with multiple coatings to further increase tool life. Although improved materials and coatings for cutting inserts have increased tool life to some degree, even the best cutting inserts must be replaced frequently, particularly in machining titanium and similar super alloy materials.
In addition to the improved materials and coatings used in the manufacture of cutting inserts, attempts have been made to increase tool life by reducing the temperature of the cutting insert and chip, and lubricating the cutting insert-workpiece interface. One method of cooling and lubricating has been to employ a quenching operation in which the tool holder and workpiece are flooded with a low pressure stream of any one of a number of types of coolant. Typically, a nozzle is disposed several inches above the cutting tool and workpiece which directs a low pressure stream of coolant onto the workpiece, tool holder and on top of the chips being produced. This technique, known as flood cooling, effectively cools only the upper surface of the chips, and that portion of the tool holder near the edge of the socket in which the cutting insert is mounted.
The underside of the chip which makes contact with the cutting insert, and the interface between the cutting insert and workpiece, are not cooled by a low pressure stream of coolant directed from above the tool holder. This is because the heat produced in the area of the chip and the cutting edge of the insert, particularly at the high operating speeds of modern milling machine tools or turning machines, vaporizes the coolant well before it can flow near the cutting edge of the insert.
In addition to the ineffectiveness of flood cooling, it can result in thermal failure of the cutting inserts. This occurs because a high temperature gradient is developed between the very hot area immediately surrounding the cutting edge of the insert, and the cooler inner portion of the insert mounted in the socket of the tool holder. The coolant cannot reach the cutting edge of the insert before it is vaporized and thus effectively cools only the area of the insert which is held in the tool holder. This extreme difference in temperature between the cutting edge and the remainder of the cutting insert can result in thermal failure.
One attempt to improve prior art flood cooling methods is found in U.S. Pat. No. 2,653,517 to Pigott. This patent teaches a method and apparatus for applying cooling liquids at a velocity of approximately 260 feet per second to a location between the workpiece and the back or rear edge of the insert beneath the top or exposed surface of the insert where the chip is produced. This general approach is also disclosed in German Pat. No. 3,004,166. The problem with such methods of cooling is that the coolant is not introduced at a location where cratering and high temperatures of the cutting insert occur; that is, between the exposed, top surface of the insert and the bottom surface of the chip produced. The introduction of coolant underneath the cutting insert does little or nothing to reduce the frictional engagement between the chip and cutting insert.
An alternative to the flood cooling methods described above is taught in U.S. Pat. No. 4,302,135 to Lillie. The rotary cutting tool disclosed in the Lillie patent comprises a body formed with a shank portion and a cutting portion through which a longitudinal bore extends including an inlet in the shank portion and an outlet in the cutting portion. Formed in the bottom surface of the cutting portion and extending radially outwardly from the outlet are spaced channels which terminate at sockets adapted to mount cutting inserts. The channels align with grooves formed in the cutting inserts which lead to the cutting edge of the inserts. Coolant is pumped through the central passageway, directed radially outwardly through the channels and is then radially deflected by the tool body along the coolant flow channels in the cutting inserts.
An attempt is made with the Lillie rotary tool to direct a high velocity, high pressure coolant flow to the area of the cutting insert-workpiece interface. But the structure provided in the Lillie patent does not permit the coolant to reach the cutting edge--workpiece interface, or the chips in that immediate area, due to the intense heat developed by the cutting operation. Flow of the coolant from the central passageway in Lillie to the cutting inserts is essentially unconfined or open to atmospheric pressure. Once the coolant is ejected from the outlet in the central passageway, its pressure and velocity drop by an order of magnitude. This is because the cross-sectional area of the base of the cutting portion of the Lillie tool is relatively large compared to that of the central bore, and the coolant flow is exposed to the atmosphere as each cutting insert rotates to the area already cut by the tool. With its pressure and velocity substantially reduced, the coolant stream provided in Lillie simply vaporizes before it can reach the immediate area of the cutting edge-workpiece interface where the intense heat is produced. The Lillie invention is thus essentially a flood cooling system in which any cooling achieved is confined to an area of the cutting insert immediately adjacent the end of the radial channels in the cutting portion of the body, and/or to the chips flowing outwardly from the cutting edge-workpiece interface when they reach that location.
In addition to limited tool life, another pervasive problem in the cutting tool industry involves the proper breakage and removal of chips from the area of the cutting insert and holder. Preferably, chips should be broken into short segments when sheared from the workpiece. If they are not broken but form in a continuous length, the chips tend to wrap around the cutting insert, tool holder and/or the workpiece which can lead to tool failure or at least require periodic interruption of the machining operation to clear the area of impacted or bundled chips.
Current attempts to solve the chip breaking and removal problem are limited to various designs of cutting inserts having a chip breaker groove, which is a groove formed in the top surface of the insert immediately adjacent the cutting edge. Chip breaker grooves engage the chips as they shear from the workpiece and turn or bend them upwardly from the surface of the insert so that they tend to fracture. While acceptable performance has been achieved with some chip breaker groove designs in some applications, variables in machining operations such as differing materials, types of machines, depths of cuts, feed rates and speeds make it virtually impossible for one chip breaker groove design to be effective in all applications. This is evidenced by the multitude of chip breaker designs now available which are intended to accommodate the widely varying machining conditions which can occur in industry. Selection of a suitable cutting insert for a particular application, if one exists at all, can be an expensive, difficult and continuous problem.